(1) Field of the Invention
This invention relates to an optical multiplexer/demultiplexer including an arrayed waveguide grating and a waveguide type optical coupler including a sector slab waveguide.
(2) Description of the Related Art
With an explosive increase in data traffic on networks, in recent years attention has been riveted to photonic networks on which a large amount of data can be transferred. To realize such networks, wavelength division multiplexing (WDM) optical communication networks are being built. An arrayed waveguide grating (AWG) in which the technology of a planar lightwave circuit (PLC) is adopted is a likely candidate for an optical wavelength multiplexer/demultiplexer essential to these WDM transmission systems.
FIG. 17 is a view showing the structure of a conventional arrayed waveguide grating.
As shown in FIG. 17, an arrayed waveguide grating 200 has the following waveguide structure. A sectors lab waveguide 203 is connected to the output side of one or more optical input waveguides 202 arranged via a mode conversion waveguide 207 described later. An arrayed waveguide 204 is connected to the input side of the sector slab waveguide 203. A sector slab waveguide 205 is connected to the output side of the arrayed waveguide 204. A plurality of optical output waveguides 206 are connected to the output side of the sector slab waveguide 205. Usually the arrayed waveguide grating 200 is made by forming the above waveguide structure on, for example, a silicon substrate with cores made from, for example, siliceous glass.
The sector slab waveguide 203 on the input side has the center of curvature at the end of the middle waveguide of the optical input waveguides 202. The sector slab waveguide 205 on the output side also has the center of curvature at the end of the middle waveguide of optical output waveguides 206. The sector slab waveguides 203 and 205 have a structure in which the optic axes of waveguides in the arrayed waveguides 204 are located radially from the center of curvature. As a result, the optical arrangement of the sector slab waveguide 203 and arrayed waveguides 204 and of the sector slab waveguide 205 and arrayed waveguides 204 are the same as that of a concave mirror. That is to say, they will function the same as a lens. Moreover, in the arrayed waveguides 204, there is optical path difference xcex94L between any two adjacent waveguides.
If the arrayed waveguide grating 200 functions as an optical demultiplexer, optical signal with a plurality of wavelengths is multiplexed by a WDM system and is input from the optical input waveguides 202 to the sector slab waveguide 203 via the mode conversion waveguide 207. This wavelength-multiplexed optical signal spreads in the sector slab waveguide 203 by diffraction and is spreaded to each of the waveguides in the arrayed waveguide 204. In this case, the phases of optical signal spreaded to the waveguides in the arrayed waveguide 204 are the same.
The optical signal beams which propagated through the arrayed waveguide 204 are given phase difference corresponding to optical path difference xcex94L between adjacent waveguides, interfere with one another in the sector slab waveguide 205 on the output side, and are condensed into the optical output waveguides 206. In this case, phase difference given in the arrayed waveguide 204 depends on wavelengths, so the optical signal beams are condensed into the different optical output waveguides 206 according to their wavelengths. As a result, the wavelength-multiplexed optical signal input from the optical input waveguides 202 is demultiplexed according to wavelengths and is output from the different optical output waveguides 206.
Operation in the arrayed waveguide grating 200 is reversible. That is to say, if the direction in which optical signal travels is inverted, the arrayed waveguide grating 200 will function as an optical multiplexer. Moreover, for example, the sector slab waveguide 203 alone can be used. In this case, the sector slab waveguide 203 will function as an optical coupler for spreading optical signal input from the optical input waveguides 202 into a plurality of optical output waveguides connected to its exit.
For example, the number of the optical input waveguides 202 and optical output waveguides 206 located corresponds to that of signal light beams with different wavelengths which are obtained as a result of demultiplexing by the arrayed waveguide grating 200 or which are to be multiplexed by the arrayed waveguide grating 200. Moreover, usually the arrayed waveguide 204 includes a large number of waveguides. In FIG. 17, for the sake of simplicity, only one optical input waveguide 202 is shown and the number of waveguides included in the arrayed waveguides 204 and optical output waveguide 206 is reduced.
By the way, if the arrayed waveguide grating 200 is used as an optical demultiplexer, the passband characteristic of optical signal obtained in each optical output waveguide 206 are as follows. The intensity of optical signal obtained in each optical output waveguide 206 is the highest at center wavelength xcex0 and becomes significantly lower at a wavelength farther from the center wavelength xcex0. In actual optical communication systems, however, the passband characteristic of propagated signal must be flat with, for example, fluctuations in the wavelength of the light taken into consideration so that the intensity of the signal will be constant in a moderately wide wavelength range with the center wavelength xcex0 as its center. In the conventional arrayed waveguide grating 200, therefore, the mode conversion waveguide 207 is located between the optical input waveguides 202 and sector slab waveguide 203.
FIG. 18 is an enlarged view of portions around the mode conversion waveguide 207. In FIG. 18, the shape of modes of optical signal propagating through the sector slab waveguide 203 is also shown.
As shown in FIG. 18, the mode conversion waveguide 207 is a waveguide in the shape of a paraboloid in which the width of a core widens in the direction of the exit, and connects the exit of the optical input waveguides 202 and the entrance of the sector slab waveguide 203. The mode conversion waveguide 207 includes a core in the shape of a paraboloid, so optical signal as shown by a curve 181, the shape of a mode of which has two peaks, will be output to the sector slab waveguide 203. When this optical signal propagates through the sector slab waveguide 203, the width of the shape of its mode will widen. At this time the shape of its mode can be shown by a curve 182. Then the optical signal is input to the arrayed waveguide 204.
Now, the principles underlying flattening a passband characteristic by locating the mode conversion waveguide 207 will be described with reference to FIGS. 19 and 20. FIGS. 19(A) and 19(B) are schematic views showing how optical signal output from the mode conversion waveguide 207 propagates through the arrayed waveguide grating 200. FIG. 19(A) shows how optical signal output from the mode conversion waveguide 207 propagates through portions around the sector slab waveguide 203 on the input side. FIG. 19(B) shows how optical signal output from the mode conversion waveguide 207 propagates through portions around the sector slab waveguide 205 on the output side.
In FIGS. 19(A) and 19(B), peaks P1 and P2 appear in the shape of a mode of optical signal with a wavelength of xcex1 which propagated through the optical input waveguides 202, on which a mode conversion was performed in the mode conversion waveguide 207, and which was output to the sector slab waveguide 203. The peaks P1 and P2 are input to the arrayed waveguide 204 at different positions at the exit of the sector slab waveguide 203. Therefore, after the peaks P1 and P2 are input to the sector slab waveguide 205 on the output side via the arrayed waveguide 204, they will converge at different positions X1 and X2, respectively, at the exit of the sector slab waveguide 205 shown in FIG. 19(B).
It is assumed that there is optical signal with a wavelength of xcex2 different from the above optical signal and that after the optical signal with a wavelength of xcex2 is input to the optical input waveguides 202 and then is output from the mode conversion waveguide 207, peaks Q1 and Q2 (not shown) appear in the shape of its mode. In this case, the peaks Q1 and Q2 will converge at different positions Y1 and Y2, respectively, at the exit of the sector slab waveguide 205 on the output side. Phase difference given by the arrayed waveguide 204 depends on wavelengths, so Y1 and Y2 are different from X1 and X2 respectively.
FIG. 20 is a graph for describing the passband characteristic of optical signal in the optical output waveguide 206.
A case where structural parameters are selected so that the position X1 where the peak P1 included in the optical signal with a wavelength of xcex1 converges matches the position Y2 where the peak Q2 included in the optical signal with a wavelength of xcex2 converges, that is to say, so that the peak P1 included in the optical signal with a wavelength of xcex1 and the peak Q2 included in the optical signal with a wavelength of xcex2 are input to the same waveguide in the optical output waveguide 206 will now be described. It is assumed that the passband characteristics of the peaks P1 and Q2 are given by curves 211 and 212, respectively, in FIG. 20. Then the passband characteristic of optical signal in this waveguide can be obtained by adding together the passband characteristics of the peaks P1 and Q2. This passband characteristic shown by a curve 213 indicates that the shape of a spectrum is flat in a wavelength range of from the wavelength corresponding to the peak P1 to the wavelength corresponding to the peak Q2.
If structural parameters for the arrayed waveguide grating 200 are selected properly according to a passband characteristic needed, the shape of a spectrum can be made flat in a necessary wavelength range on the above principles.
In the above mode conversion waveguide 207, however, single mode light which propagates through the optical input waveguides 202 merely branches and peaks which appear in the shape of modes of two optical beams are added together to obtain a passband characteristic on the output side. Therefore, the intensity of optical signal cannot be controlled accurately. For example, proper control which reduces cross talk in each of waveguides in the optical output waveguide 206 cannot be exercised.
FIG. 21 is a graph of passband characteristics for describing how cross talk occurs.
As shown in FIG. 21, a spectrum which is flat in a demultiplexed wavelength range in each waveguide as a center wavelength will appear in a passband characteristic obtained in the optical output waveguide 206. When a spectrum corresponding to an adjacent wavelength appears in a flat portion, cross talk will occur in this portion. Therefore, the shape of a spectrum in which the intensity of optical signal lowers sharply outside a necessary wavelength range is desirable as a passband characteristic in each waveguide.
With the above conventional structure in which the mode conversion waveguide 207 is located at the exit of the optical input waveguides 202, however, the shape of a spectrum will spread with the expansion of a flat passband, resulting in a wider cutoff region. Therefore, the passbands of adjacent wavelengths come near to each other and the spectra overlap considerably. As a result, much cross talk will occur.
As stated above, with the conventional arrayed waveguide grating 200 in which the mode conversion waveguide 207 is used, the shape of a mode of optical signal which propagates to the exit of the sector slab waveguide 203 on the input side depends on the shape of a mode of the optical input waveguides 202, so it is impossible to accurately control the shape of a mode of light generated in the sector slab waveguide 203 according to a passband characteristic needed in the optical output waveguide 206.
Moreover, if the sector slab waveguide 203 on the input side is used as an optical coupler, optical signal propagated to each waveguide connected to its exit must be uniform in intensity. Therefore, accurate control must be exercised over the shape of a mode of optical signal which propagates through the sector slab waveguide 203, but this is impossible.
The present invention was made under the background circumstances as described above. An object of the present invention is to provide an optical multiplexer/demultiplexer which can exercise accurate control over the passband characteristic of demultiplexed optical signal in a waveguide on the output side.
Another object of the present invention is to provide a waveguide type optical coupler which can exercise proper control over the shape of a mode of output optial signal.
In order to achieve the first object, an optical multiplexer/demultiplexer in which a waveguide structure comprising one or more optical input waveguides arranged, a first sector slab waveguide connected to the output side of the optical input waveguides, arrayed waveguides connected to the output side of the first sector slab waveguide and including a plurality of waveguides arranged any two adjacent waveguides of which differ in length by a constant value for propagating output optical signal, a second sector slab waveguide connected to the output side of the arrayed waveguides, and a plurality of optical output waveguides arranged and connected to the output side of the second sector slab waveguide is formed on a substrate is provided. A plurality of long and narrow guide waveguides, being areas where an effective refractive index is greater than an effective refractive index of the first sector slab waveguide, are formed in an area adjacent to the first sector slab waveguide in this optical multiplexer/demultiplexer so that the plurality of long and narrow guide waveguides will branch at the center of curvature of the first sector slab waveguide and so that the plurality of long and narrow guide waveguides will extend in an output direction without intersecting with one another.
Moreover, in order to achieve the second object of the present invention, a waveguide type optical coupler comprising a sector slab waveguide that spreads optical signal input from one or more optical input waveguides arranged and connected to the entrance of the sector slab waveguide to a plurality of optical output waveguides arranged and connected to the output side of the sector slab waveguide is provided. A plurality of long and narrow guide waveguides, being areas where an effective refractive index is greater than an effective refractive index of the sector slab waveguide, are formed in the core or in an area adjacent to the sector slab waveguide in this waveguide type optical coupler so that the plurality of long and narrow guide waveguides will branch at the center of curvature of the sector slab waveguide and so that the plurality of long and narrow guide waveguides will extend in an output direction without intersecting with one another.
The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.